Run ProcessImplicitDefs on SSA form where it can be much simpler.

Implicitly defined virtual registers can simply have the <undef> bit set
on all uses, and copies can be turned into implicit defs recursively.

Physical registers are a bit trickier. We handle the common case where a
physreg def is used by a nearby instruction in the same basic block. For
more complicated cases, just leave the IMPLICIT_DEF instruction in.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@159149 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/Passes.cpp

@@ -572,6 +572,8 @@ void TargetPassConfig::addFastRegAlloc(FunctionPass *RegAllocPass) {
 /// optimized register allocation, including coalescing, machine instruction
 /// scheduling, and register allocation itself.
 void TargetPassConfig::addOptimizedRegAlloc(FunctionPass *RegAllocPass) {
+  addPass(ProcessImplicitDefsID);
+
   // LiveVariables currently requires pure SSA form.
   //
   // FIXME: Once TwoAddressInstruction pass no longer uses kill flags,
@@ -590,12 +592,6 @@ void TargetPassConfig::addOptimizedRegAlloc(FunctionPass *RegAllocPass) {
   }
   addPass(TwoAddressInstructionPassID);
 
-  // FIXME: Either remove this pass completely, or fix it so that it works on
-  // SSA form. We could modify LiveIntervals to be independent of this pass, But
-  // it would be even better to simply eliminate *all* IMPLICIT_DEFs before
-  // leaving SSA.
-  addPass(ProcessImplicitDefsID);
-
   if (EnableStrongPHIElim)
     addPass(StrongPHIEliminationID);